1. Field of the Invention
The present invention relates to a reversible recording material based on thermal energy, and more particularly to the reversible recording material comprising cholesteric liquid crystal compounds.
2. Description of the Related Art
For reversible recording media, many attempts have been made in recent years. Among these, Japanese Laid-Open Patent Application No. 11-24027 describes a reversible thermal recording material comprising a cholesteric liquid crystal compound having a molecular weight of less than 2000 and a glass transition temperature of more than 35xc2x0 C., or a mixture thereof.
As shown in FIG. 1, the recording material comprising the cholesteric liquid crystal compound is placed between two substrates, one of the two substrates being provided with a light-absorbing layer on one side of the substrate, so as to form a reversible display device. The cholesteric liquid crystal compound in a cholesteric liquid crystal phase exhibits typical iridescent colors. When the compound in the cholesteric liquid crystal phase is rapidly cooled by a suitable cooling means, the compound solidifies while keeping an iridescent color observed at a temperature where the rapid cooling started, thereby allowing a reflecting color from the cholesteric liquid crystal compound to be fixed. A desired reflecting color can be preserved by selecting the cooling commencement temperature of the cholesterlc liquid crystal phase. In this way, a variety of colors can be recorded stably at the room temperature.
Further, when the above solidified compound is heated to the cholesteric liquid crystal phase, that is to say, erasure is carried out, writing to the compound can be achieved again (See, N. Tamaoki et al., Adv. Mater., 1997, 9, 1102-1104). Thus, the above chloesteric liquid crystal compound can be applied for rewritable full-color recording and also for high density memory media.
Although the above compound has a promising feature for the reversible recording material, there is a problem relating to inability to provide the reflecting light having a long wavelength. To solve this problem, Japanese Laid-Open Patent Application Nos. 6-258622 and 8-152605 disclose a liquid crystal mixture comprising more than two cholesteric liquid crystal compounds dispersed in a polymer as the recording media. In this recording media, there may be occasions that a phase separation between the liquid crystal compound and the polymer occurs to fail to widen a wavelength range of the reflecting light.
Accordingly, it is an object of the present invention to provide a reversible recording material based on thermal energy in which the disadvantage of the aforementioned prior art is eliminated.
The inventors of the present invention have performed much research and investigation regarding a material for thermal reversible recording in which the faults described for the conventional technologies have been eliminated.
The above object of the present invention is achieved by a reversible recording material which is sensitive to heat, comprising a liquid crystal mixture comprising at least two cholesteric liquid crystal compounds, the at least two cholesteric liquid crystal compounds being compatible each other, wherein after at least a part of the reversible recording material is heated to a temperature where the at least a part of the reversible recording material becomes a cholesteric liquid crystal phase, the at least a part of the reversible recording material is rapidly cooled down to below a glass transition temperature thereof, so that a reflecting color from the reversible recording material can be fixed to achieve a recording.
The above object of the present invention is also achieved by a reversible recording material which is sensitive to heat, comprising a liquid crystal mixture comprising at least two cholesteric liquid crystal compounds, the at least two cholesteric liquid crystal compounds being compatible each other, wherein after at least a part of the reversible recording material is heated to a temperature where the at least a part of the reversible recording material becomes an isotropic phase, the at least a part of the reversible recording material is cooled down to below a glass transition temperature thereof at a predetermined cooling rate based on a reflecting color to be fixed, so that the reflecting color from the reversible recording material can be fixed to achieve a recording.
According to the present invention, at least one cholesteric liquid crystal compound forming the reversible recording material has a molecular weight of less than 2000 and a glass transition temperature equal to or more than 30xc2x0 C.
The present invention provides a reversible recording material by which a reflecting color (light) having the wide wavelength range can be obtained without crystallization during a cooling process.
Furthermore, according to the present invention, it is possible to preserve the reflecting color at the room temperature for a long period.